We began our research comparing
plastic and wooden cutting boards after the U.S. Department of
Agriculture told us they had no scientific evidence to support
their recommendation that plastic, rather than wooden cutting
boards be used in home kitchens. Then and since, the U.S.
Department of Agriculture's Meat and Poultry Inspection Manual
(official regulations) and the U.S. Food and Drug
Administration's 1999 Food Code (recommended regulations for
restaurants and retail food sales in the various states of the
U.S.) permit use of cutting boards made of maple or similar
close-grained hardwood. They do not specifically authorize
acceptable plastic materials, nor do they specify how plastic
surfaces must be maintained.

Our research was first
intended to develop means of disinfecting wooden cutting surfaces
at home, so that they would be almost as safe as plastics. Our
safety concern was that bacteria such as Escherichia coli O157:H7
and Salmonella, which might contaminate a work surface when raw
meat was being prepared, ought not remain on the surface to
contaminate other foods that might be eaten without further
cooking. We soon found that disease bacteria such as these were
not recoverable from wooden surfaces in a short time after they
were applied, unless very large numbers were used. New plastic
surfaces allowed the bacteria to persist, but were easily cleaned
and disinfected. However, wooden boards that had been used and
had many knife cuts acted almost the same as new wood, whereas
plastic surfaces that were knife-scarred were impossible to clean
and disinfect manually, especially when food residues such as
chicken fat were present. Scanning electron micrographs revealed
highly significant damage to plastic surfaces from knife cuts.

Although the bacteria that
have disappeared from the wood surfaces are found alive inside
the wood for some time after application, they evidently do not
multiply, and they gradually die. They can be detected only by
splitting or gouging the wood or by forcing water completely
through from one surface to the other. If a sharp knife is used
to cut into the work surfaces after used plastic or wood has been
contaminated with bacteria and cleaned manually, more bacteria
are recovered from a used plastic surface than from a used wood
surface.

"Manual cleaning" in
our experiments has been done with a sponge, hot tapwater, and
liquid dishwashing detergent. Mechanical cleaning with a
dishwashing machine can be done successfully with plastic
surfaces (even if knife-scarred) and wooden boards especially
made for this. Wooden boards, but not plastics, that are small
enough to fit into a microwave oven can be disinfected rapidly,
but care must be used to prevent overheating. Work surfaces that
have been cleaned can be disinfected with bleach (sodium
hypochlorite) solutions; this disinfection is reliable only if
cleaning has been done successfully.

The experiments described have
been conducted with more than 10 species of hardwoods and with 4
plastic polymers, as well as hard rubber. Because we found
essentially no differences among the tested wood species, not all
combinations of bacteria and wood were tested, nor were all
combinations of bacteria and plastics or hard rubber. Bacteria
tested, in addition to those named above, include Campylobacter
jejuni, Listeria monocytogenes, and Staphylococcus aureus.

We believe that the
experiments were designed to be properly representative of
conditions in a home kitchen. They may or may not be applicable
to other plastic and wooden food contact surfaces or to cutting
boards in commercial food processing or food service operations,
but we have no reason to believe that they are not relevant,
except that not all plastic surfaces are subject to
knife-scarring. Before our first studies had been published, they
were criticized incorrectly for not having included used
(knife-scarred) cutting surfaces. We had been careful to include
used surfaces, and so were surprised that others who did later
experiments and claimed to have refuted our findings often had
used only new plastic and wood. Although some established
scientific laboratories say their results differ from ours, we
have received multiple communications from school children who
have done science projects that have reached essentially the same
conclusions that we did.

We have no commercial
relationships to any company making cutting boards or other food
preparation utensils. We have tested boards and cleaning and
disinfection products, some of which were supplied to us gratis.
We have not tested all of the products that have been sent to us,
simply because there is not time. We are aware that there are
other food preparation surfaces made of glass or of stainless
steel; we have done very little with these because they are quite
destructive of the sharp cutting edges of knives, and therefore
introduce another class of hazard to the kitchen. We believe, on
the basis of our published and to-be-published research, that
food can be prepared safely on wooden cutting surfaces and that
plastic cutting surfaces present some disadvantages that had been
overlooked until we found them.

In addition to our laboratory
research on this subject, we learned after arriving in California
in June of 1995 that a case-control study of sporadic
salmonellosis had been done in this region and included cutting
boards among many risk factors assessed (Kass, P.H., et al.,
Disease determinants of sporadic salmonellosis in four northern
California counties: a case control study of older children and
adults. Ann. Epidemiol. 2:683-696, 1992.). The project had been
conducted before our work began. It revealed that those using
wooden cutting boards in their home kitchens were less than half
as likely as average to contract salmonellosis (odds ratio 0.42,
95% confidence interval 0.22-0.81), those using synthetic
(plastic or glass) cutting boards were about twice as likely as
average to contract salmonellosis (O.R. 1.99, C.I. 1.03-3.85);
and the effect of cleaning the board regularly after preparing
meat on it was not statistically significant (O.R. 1.20, C.I.
0.54-2.68). We know of no similar research that has been done
anywhere, so we regard it as the best epidemiological evidence
available to date that wooden cutting boards are not a hazard to
human health, but plastic cutting boards may be.